1. Field of the Invention
The present invention relates to an improvement of an engine decompression system in which a decompression cam shaft is provided on a valve operating cam shaft equipped with a valve operating cam for opening and closing an engine valve or is provided on a rotating member integrally connected to the valve operating cam shaft. The decompression cam shaft is capable of rotating between an operating position in which a decompression cam projects above a base face of the valve operating cam so as to slightly open the engine valve during an engine compression stroke, and a release position in which the decompression cam is withdrawn beneath the base face so as to allow the engine valve to close. A centrifugal mechanism is connected to the decompression cam shaft to maintain the decompression cam shaft at the operating position in an engine starting rotational region, and to rotate the decompression cam shaft to the release position in a normal running region.
2. Description of the Related Art
Engine decompression systems are already known as disclosed in, for example, Japanese Utility Model Registration Publication No. 51-41974. In such a conventional engine decompression system, rotation of a decompression cam shaft from an operating position to a release position is proportionally controlled according to an increase in the rotational speed of the valve operating cam shaft.
However, in the engine decompression system, in order to minimize the cranking load when starting the engine, it is desirable that the projection height of a decompression cam from a base face of a valve operating cam is relatively large, and also in order to stabilize a complete combustion state in the engine, it is desirable that the projection height of the decompression cam is decreased, so that it is difficult for the conventional centrifugal mechanism to satisfy such decompression characteristics.